Orbital Mechanics and Applications
2. Types of Orbits
Learning Outcomes
- I can distinguish between circular and elliptical orbits.
- I can identify common satellite orbits.
- I can compare low-Earth, geostationary, and polar orbits.
- I can explain the advantages of different orbit types.
- I can relate orbit selection to mission objectives.
- Characteristics of orbit types
- Specific examples (e.g., GPS satellites)
Types of Orbits
An orbit is the curved path followed by an object as it moves under the influence of gravity. Artificial satellites can be placed into many different types of orbits depending on what the satellite is designed to do.
Two important descriptions of an orbit are its shape and its orientation.
By shape, an orbit may be:
- circular
- elliptical
By altitude and orientation, common Earth satellite orbits include:
- Low Earth Orbit (LEO)
- Medium Earth Orbit (MEO)
- Geostationary Earth Orbit (GEO)
- polar orbit
- Sun-synchronous orbit
There is no single "best" orbit. Engineers choose an orbit based on the mission objective.
Circular Orbits
A circular orbit has a constant orbital radius.
The satellite remains the same distance from the centre of the planet throughout its orbit.
For an ideal circular orbit:
- orbital radius remains constant
- orbital speed remains constant
- gravitational potential energy remains constant
- kinetic energy remains constant
- total mechanical energy remains constant
However, velocity still changes because its direction changes continuously.
Circular Orbital Speed
For a circular orbit:
v = √(GM/r)
where:
- v = orbital speed
- G = gravitational constant
- M = mass of the central body
- r = orbital radius
At a particular orbital radius, there is a specific speed required for an ideal circular orbit.
Why Circular Orbits Are Useful
Circular or nearly circular orbits are useful when engineers want predictable:
- altitude
- orbital speed
- communication distance
- observation conditions
- coverage patterns
Many communication, navigation and Earth-observation satellites use approximately circular orbits.
Elliptical Orbits
An elliptical orbit has the shape of an ellipse.
The central body lies at one focus of the ellipse rather than at its geometric centre.
The satellite's distance from the central body changes continuously.
Periapsis and Apoapsis
The closest point of an orbit to the central body is called the:
periapsis
The farthest point is called the:
apoapsis
For an Earth orbit, these have special names:
perigee = closest point to Earth
apogee = farthest point from Earth
For an orbit around the Sun:
perihelion = closest point to the Sun
aphelion = farthest point from the Sun
Speed Changes in an Elliptical Orbit
An object in an elliptical orbit does not travel at constant speed.
It moves:
fastest at periapsis
and:
slowest at apoapsis
This occurs because gravitational potential energy and kinetic energy continually transform into one another.
Near periapsis:
- gravitational potential energy is lower
- kinetic energy is greater
- speed is greater
Near apoapsis:
- gravitational potential energy is higher
- kinetic energy is smaller
- speed is lower
Energy in an Elliptical Orbit
If no non-conservative forces do work:
E_total = Eₖ + Eₚ = constant
As the satellite moves inward:
Eₚ decreases → Eₖ increases
As it moves outward:
Eₖ decreases → Eₚ increases
Therefore, speed changes naturally throughout the orbit.
Circular Versus Elliptical Orbits
| Feature | Circular Orbit | Elliptical Orbit |
|---|---|---|
| Shape | Circle | Ellipse |
| Orbital radius | Constant | Changes |
| Speed | Constant | Changes |
| Closest/farthest points | Same distance | Periapsis and apoapsis |
| GPE | Constant | Changes |
| KE | Constant | Changes |
| Central body | At centre | At one focus |
A circle can actually be considered a special case of an ellipse with zero eccentricity.
Orbital Eccentricity
The shape of an orbit can be described using eccentricity, e.
For a circular orbit:
e = 0
For an elliptical orbit:
0 < e < 1
A small eccentricity means the orbit is nearly circular.
An eccentricity closer to 1 means the ellipse is more stretched.
Why Elliptical Orbits Are Useful
Elliptical orbits can be useful when a spacecraft needs to:
- spend more time over a particular region
- travel between different orbital altitudes
- reach another planet
- transfer between circular orbits
- perform scientific observations at different distances
Transfer trajectories between orbits are often elliptical.
Classifying Earth Satellite Orbits
Earth satellites can also be classified according to:
- altitude
- inclination
- orbital period
- direction of travel
- relationship with Earth's rotation
Some of the most important categories are:
LEO, MEO, GEO and polar orbit.
These categories can overlap.
For example, a satellite can simultaneously be:
low-Earth + polar + Sun-synchronous + nearly circular.
Low Earth Orbit
Low Earth Orbit (LEO) refers broadly to relatively low-altitude Earth orbits.
LEO extends from just above the atmosphere to roughly:
2000 km altitude
Many satellites operate at a few hundred kilometres above Earth.
Characteristics of LEO
LEO satellites generally have:
- relatively low altitude
- high orbital speed
- short orbital periods
- relatively short communication distances to Earth
- relatively small coverage area at any instant
Typical orbital speeds are around:
7–8 km/s
Many complete an orbit in roughly:
90–130 minutes
depending on altitude.
Advantages of LEO
Low Earth orbit offers several important advantages.
Because satellites are relatively close to Earth:
- images can have high spatial resolution
- communication signals travel shorter distances
- signal delay can be relatively low
- less launch energy is required than for very high Earth orbits
- smaller transmitters may be sufficient for some applications
Disadvantages of LEO
A single LEO satellite covers only part of Earth at one time.
It also moves rapidly across the sky.
Therefore:
- continuous coverage may require many satellites
- ground stations must track moving satellites
- satellites may frequently move below the horizon
- atmospheric drag can affect lower LEO satellites
- orbital decay may occur
Large satellite constellations can overcome some of these limitations.
Common Uses of LEO
LEO is commonly used for:
- Earth observation
- weather monitoring
- scientific research
- crewed spaceflight
- remote sensing
- some communication systems
- imaging
- environmental monitoring
The International Space Station is also in low Earth orbit.
Medium Earth Orbit
Medium Earth Orbit (MEO) lies above LEO and below geostationary orbit.
There is a broad range of possible MEO altitudes.
One particularly important application is:
satellite navigation.
Why Navigation Satellites Use MEO
Navigation systems need satellites to:
- cover large areas
- remain visible for substantial periods
- provide signals from several directions
- form reliable global constellations
MEO provides a useful compromise between:
- coverage
- signal strength
- orbital period
- number of satellites required
MEO Compared with LEO
A MEO satellite is farther from Earth.
Therefore:
orbital speed is lower
and:
orbital period is longer
It can also see a larger fraction of Earth's surface at once.
However, communication distances are greater.
Geostationary Earth Orbit
A geostationary orbit is a special circular orbit in which a satellite appears to remain above approximately the same point on Earth's equator.
Its altitude is approximately:
35,786 km above Earth's surface
Its orbital radius is approximately:
42,164 km from Earth's centre
Requirements for a Geostationary Orbit
A geostationary satellite must:
- follow an approximately circular orbit
- orbit above Earth's equator
- travel in the same direction as Earth's rotation
- have an orbital period equal to Earth's sidereal rotation period
These conditions allow the satellite to remain above approximately the same longitude.
Why GEO Satellites Appear Stationary
Earth rotates eastward.
A geostationary satellite also orbits eastward.
If their angular speeds match, the satellite remains approximately above the same location relative to Earth's surface.
To an observer on Earth, the satellite appears nearly stationary in the sky.
Advantages of Geostationary Orbit
The nearly fixed position is extremely useful for communication.
A ground antenna can point toward the satellite and does not need to continuously track it.
A GEO satellite can also view a very large fraction of Earth's surface.
These properties make GEO useful for:
- communications
- television broadcasting
- weather monitoring
- telecommunications
- continuous regional observation
Weather Satellites in GEO
A geostationary weather satellite can continuously observe the same large region.
This is valuable for tracking:
- tropical cyclones
- cloud systems
- major storms
- atmospheric motion
- developing weather patterns
Disadvantages of GEO
Geostationary satellites are very far from Earth.
This creates several disadvantages:
- greater launch energy is required
- signals travel much farther
- communication latency is greater
- imaging resolution can be lower for a given instrument size
- coverage near the poles is poor
Therefore, GEO is not ideal for every mission.
Polar Orbits
A polar orbit passes approximately over Earth's north and south polar regions.
Instead of orbiting around the equator, the orbital plane is highly inclined relative to Earth's equator.
Earth Rotates Beneath a Polar Satellite
As a polar satellite travels around Earth, Earth rotates beneath its orbital path.
On the next orbit, the satellite passes over a different strip of Earth's surface.
Over many orbits, the satellite can observe almost the entire planet.
This makes polar orbits particularly useful for global observation.
Advantages of Polar Orbits
Polar or near-polar orbits are excellent for:
- mapping
- environmental monitoring
- ice monitoring
- land-use studies
- climate research
- ocean observation
- reconnaissance
- global Earth imaging
Unlike a geostationary satellite, a polar satellite can observe high-latitude and polar regions very effectively.
Disadvantages of Polar Orbits
A single polar satellite does not continuously remain over one location.
Instead, it repeatedly passes over different areas.
Therefore:
- continuous observation of one region is difficult
- ground stations must wait for passes
- continuous communication may require a constellation or relay network
Sun-Synchronous Orbits
A Sun-synchronous orbit (SSO) is a special type of near-polar orbit.
Its orbital plane slowly rotates so that the satellite passes over a location at approximately the same local solar time on successive passes.
Why Sun-Synchronous Orbits Are Useful
Suppose a satellite photographs the same city on different days.
If every image is taken at roughly the same local solar time, then:
- Sun angle is similar
- shadows are more consistent
- lighting conditions are easier to compare
This is extremely useful for studying changes over time.
Uses of Sun-Synchronous Orbits
Sun-synchronous orbits are widely used for:
- Earth imaging
- environmental monitoring
- agriculture
- forestry
- mapping
- ice studies
- disaster monitoring
- climate observations
SSO is therefore especially important for remote-sensing satellites.
Equatorial Orbits
An equatorial orbit lies close to Earth's equatorial plane.
Its inclination is close to:
0°
Geostationary orbit is one important example.
Equatorial orbits can be useful when missions focus on:
- equatorial regions
- communications
- low-inclination operations
Inclined Orbits
Not every orbit is equatorial or polar.
An inclined orbit crosses the equator at an angle.
The angle between the orbital plane and Earth's equatorial plane is called the inclination.
Examples:
- 0° → equatorial orbit
- about 90° → polar orbit
- intermediate values → inclined orbit
Inclination determines which latitudes a satellite can pass over.
Inclination and Ground Coverage
A satellite generally cannot directly pass over latitudes greater than its orbital inclination.
For example, a satellite with:
30° inclination
can pass over regions between approximately:
30° N and 30° S
A near-polar satellite can pass over almost every latitude.
Therefore, inclination is a major part of mission planning.
Prograde and Retrograde Orbits
A prograde orbit travels in the same general direction as Earth's rotation.
A retrograde orbit travels in the opposite direction.
Most launches use prograde orbits when possible because Earth's rotation provides some initial eastward velocity.
This can reduce launch energy requirements.
Sun-synchronous satellites, however, commonly use slightly retrograde near-polar orbits to achieve the required orbital-plane precession.
Highly Elliptical Orbits
Some satellites use highly elliptical orbits (HEO).
These have:
- low perigee
- very high apogee
- large eccentricity
Because satellites move slowly near apogee, they can spend a large fraction of each orbit over a particular region.
High-Latitude Communications
Geostationary satellites appear low on the horizon when viewed from very high latitudes.
This can make GEO communication difficult in polar regions.
Highly elliptical orbits can provide better viewing geometry for high-latitude regions.
A satellite can spend many hours near its high-altitude apogee over the desired hemisphere.
Molniya-Type Orbits
A well-known example of a highly elliptical orbit is a Molniya-type orbit.
Such orbits are designed so that satellites spend a large amount of time over high northern latitudes.
Several satellites can be used together to provide repeated regional coverage.
This illustrates how an elliptical orbit can be deliberately chosen because its changing orbital speed is useful.
Transfer Orbits
Elliptical orbits are also commonly used to move spacecraft between circular orbits.
A transfer orbit connects an initial orbit to a different orbit.
One important example is the:
Hohmann transfer orbit
Hohmann Transfer
Suppose a spacecraft begins in a low circular orbit and needs to reach a higher circular orbit.
A simplified Hohmann transfer works like this:
- spacecraft begins in lower circular orbit
- engine fires forward
- spacecraft enters an elliptical transfer orbit
- spacecraft climbs toward the higher orbit
- spacecraft reaches apogee
- another engine burn establishes the higher circular orbit
The transfer ellipse touches both circular orbits.
Geostationary Transfer Orbit
Satellites intended for geostationary orbit may first be placed into an elliptical geostationary transfer orbit (GTO).
The spacecraft then raises and circularizes its orbit near geostationary altitude.
This avoids requiring the launch vehicle to place the spacecraft directly into its final circular GEO in a single step.
Comparing LEO, GEO and Polar Orbits
These orbit descriptions represent different characteristics.
LEO primarily describes altitude.
GEO describes a very specific combination of:
- altitude
- period
- inclination
- direction
Polar describes orbital inclination.
Therefore, LEO and polar are not mutually exclusive.
A satellite can be both:
LEO and polar
at the same time.
Major Comparison
| Feature | LEO | GEO | Polar |
|---|---|---|---|
| Main classification | Altitude | Special orbit | Inclination |
| Typical altitude | Below ~2000 km | ~35,786 km | Often LEO |
| Orbital speed | Relatively high | Lower | Often relatively high |
| Period | Usually ~90–130 min | ~1 sidereal day | Often ~90–110 min |
| Coverage per satellite | Smaller | Very large | Global over time |
| Same region continuously? | No | Yes | No |
| Polar coverage | Depends on inclination | Poor | Excellent |
| Signal delay | Relatively low | Greater | Relatively low if LEO |
| Typical uses | Imaging, science, communications | Communications, weather | Mapping, Earth observation |
Orbit Selection Depends on the Mission
Engineers do not choose an orbit simply because one orbit is "better."
They ask:
What must the satellite accomplish?
Different objectives lead to different orbital choices.
Mission: High-Resolution Earth Imaging
Suppose a satellite must photograph Earth's surface in high detail.
A lower altitude generally allows better spatial resolution for a given imaging system.
A suitable choice might be:
LEO
often combined with:
polar or Sun-synchronous orientation
This allows detailed observations across much of Earth.
Mission: Continuous Weather Monitoring
Suppose meteorologists want to continuously watch a large region.
A good choice is:
GEO
The satellite can remain above approximately the same region and repeatedly observe developing weather systems.
Mission: Global Environmental Mapping
Suppose scientists want to map:
- forests
- glaciers
- oceans
- land-use changes
A good choice is often:
polar or Sun-synchronous LEO
Earth rotates beneath the satellite, allowing global coverage over time.
Mission: Satellite Navigation
A navigation system requires:
- broad coverage
- several satellites visible simultaneously
- predictable orbital geometry
A common solution is:
MEO constellations
This provides a useful compromise between coverage and satellite number.
Mission: High-Latitude Communications
Geostationary satellites can be difficult to use at very high latitudes because they appear close to the horizon.
A highly elliptical orbit can provide better high-latitude coverage.
The satellite spends substantial time near apogee over the target region.
Mission: Human Spaceflight
Crewed Earth-orbiting missions often use LEO because:
- launch requirements are lower than for distant orbits
- communication delays are small
- Earth is relatively close
- emergency return is more practical
- resupply is easier than in higher orbits
This does not make LEO easy—it simply makes it more accessible than many alternatives.
Satellite Constellations
One satellite may not provide enough coverage.
A satellite constellation consists of multiple satellites arranged in coordinated orbits.
Constellations can provide:
- continuous global communications
- navigation
- frequent Earth imaging
- weather observations
- backup if individual satellites fail
LEO Constellations
A single LEO satellite moves quickly relative to Earth's surface.
Continuous communication therefore requires many satellites.
As one satellite moves below the horizon, the user's connection can be transferred to another satellite.
This is called a handover.
The disadvantage is that large numbers of satellites and complex ground systems may be required.
GEO Coverage
A GEO satellite sees a much larger portion of Earth at once.
A relatively small number of GEO satellites can provide broad coverage across much of the planet.
However, polar coverage remains poor.
The much greater signal distance also creates greater communication delay.
Signal Travel Time
Electromagnetic signals travel at approximately:
3.0 × 10⁸ m/s
A signal sent to a satellite must travel:
- from Earth to the satellite
- often back to Earth
A GEO satellite is tens of thousands of kilometres away.
A LEO satellite may be only hundreds of kilometres away.
Therefore:
LEO generally provides lower latency than GEO.
This can matter for:
- voice communication
- video calls
- interactive internet
- remote control systems
Orbital Period and Mission Design
Orbital period strongly affects how often a satellite passes over a location.
Short-period LEO satellites may make many revolutions each day.
GEO satellites complete approximately one orbit per sidereal day.
Highly elliptical satellites may be designed so their periods and orientations produce repeated coverage of selected regions.
Therefore, orbital period is not merely a consequence of the orbit—it is often part of the mission design.
Changing an Orbit
A satellite can change its orbit by firing its engines.
An engine burn changes the satellite's:
velocity
which changes its:
orbital energy
and therefore its trajectory.
A forward burn can raise part of the orbit.
A backward burn can lower part of the orbit.
Further burns can circularize the new orbit.
Orbit Raising
Suppose a satellite begins in a low circular orbit.
A forward engine burn increases its speed.
The satellite enters an ellipse whose apogee is higher.
At apogee, another forward burn can raise the perigee and create a higher circular orbit.
Although the final higher circular orbit has a lower orbital speed, energy had to be added to reach it.
Orbit Lowering
A spacecraft can fire its engine opposite its direction of travel.
This decreases its orbital energy.
The opposite side of the orbit drops.
This principle can be used for:
- changing operational orbits
- atmospheric re-entry
- satellite disposal
Orbital Debris and Orbit Selection
Orbit selection also involves safety.
Earth orbit contains:
- operating satellites
- inactive satellites
- spent rocket stages
- fragments from collisions or breakups
Even small objects can be dangerous because orbital speeds are several kilometres per second.
Mission planners therefore consider collision probability and debris environments when selecting and maintaining orbits.
End-of-Life Planning
Satellites cannot simply be ignored when their missions finish.
Depending on orbit, engineers may plan to:
- lower the satellite so it re-enters
- move it to a disposal orbit
- remove remaining stored energy
- reduce the chance of creating debris
Responsible orbit selection therefore includes the satellite's entire life cycle, not just its operational phase.
Choosing Between Orbit Types
Consider four different mission objectives.
Satellite A: Detailed Crop Imaging
Needs:
- detailed surface images
- repeated global coverage
- similar lighting
Suitable orbit:
Sun-synchronous LEO
Satellite B: Continuous Regional Weather Monitoring
Needs:
- constant view of one large region
Suitable orbit:
GEO
Satellite C: Global Navigation
Needs:
- large-area coverage
- multiple satellites visible
Suitable orbit:
MEO constellation
Satellite D: High-Latitude Communications
Needs:
- long visibility over northern regions
Possible solution:
highly elliptical orbit
The orbit follows from the mission requirements.
Orbit Selection as an Engineering Trade-Off
Selecting an orbit usually requires balancing competing advantages.
A lower orbit may provide:
- better image resolution
- lower signal delay
- easier launch access
but also:
- smaller coverage
- shorter visibility
- greater atmospheric drag
A higher orbit may provide:
- greater coverage
- longer visibility
but also:
- greater communication distance
- higher launch requirements
- lower image resolution for a comparable instrument
No orbit maximizes every desirable property.
A Mission-Planning Checklist
When selecting an orbit, engineers may ask:
- What area must be observed?
- Is continuous coverage required?
- What image resolution is needed?
- How often must the satellite revisit a location?
- Are polar regions important?
- How much communication delay is acceptable?
- How much launch energy is available?
- How long must the satellite operate?
- How much atmospheric drag is acceptable?
- Is a constellation required?
- What debris risks exist?
- What happens at the end of the mission?
The answers determine which orbital characteristics are most useful.
Common Mistakes
Thinking every orbit is circular
Many natural and artificial orbits are elliptical.
Thinking the central body is at the centre of an elliptical orbit
It lies at one focus.
Thinking satellites move at constant speed in elliptical orbits
They move fastest near periapsis and slowest near apoapsis.
Thinking LEO, GEO and polar describe the same property
LEO mainly describes altitude, GEO describes a specific orbit, and polar describes inclination.
Thinking a polar satellite must be far from Earth
Many polar satellites operate in LEO.
Thinking geostationary satellites can orbit above any latitude
A true geostationary satellite must orbit above the equator.
Thinking GEO satellites provide excellent polar coverage
Their viewing geometry is poor at high latitudes.
Thinking higher satellites move faster
For circular Earth orbits, higher satellites move more slowly.
Thinking GEO means the satellite is physically stationary
It is moving at several kilometres per second; it only appears stationary relative to Earth's surface.
Thinking one orbit is best for every satellite
Orbit selection depends on mission objectives.
Key Terms
Orbit — curved gravitational trajectory of an object around another body.
Circular orbit — orbit with constant orbital radius.
Elliptical orbit — oval-shaped orbit in which orbital radius changes.
Eccentricity — numerical measure of how much an orbit differs from a circle.
Periapsis — closest point in an orbit to the central body.
Apoapsis — farthest point in an orbit from the central body.
Perigee — closest point to Earth.
Apogee — farthest point from Earth.
Low Earth Orbit (LEO) — relatively low-altitude Earth orbit, broadly below about 2000 km.
Medium Earth Orbit (MEO) — region above LEO and below geostationary altitude.
Geostationary Earth Orbit (GEO) — circular equatorial orbit in which a satellite appears approximately stationary above one longitude.
Polar orbit — highly inclined orbit passing near Earth's poles.
Sun-synchronous orbit — near-polar orbit designed to pass locations at approximately the same local solar time.
Inclination — angle between an orbital plane and a reference plane such as Earth's equator.
Prograde orbit — orbit travelling in the same general direction as the central body's rotation.
Retrograde orbit — orbit travelling opposite the central body's rotation.
Highly elliptical orbit (HEO) — orbit with a large difference between periapsis and apoapsis.
Transfer orbit — temporary orbit used to move between other orbits.
Hohmann transfer — efficient elliptical transfer between approximately circular orbits.
Satellite constellation — coordinated group of satellites providing combined coverage or services.
Orbital period — time required for one complete orbit.
Orbital debris — human-made objects or fragments remaining in space that no longer serve a useful function.
Key Takeaways
- Orbits can be classified by shape, altitude, inclination, period and orientation.
- Circular orbits have constant orbital radius and constant orbital speed.
- Elliptical orbits have changing radius and changing speed.
- The central body lies at one focus of an elliptical orbit.
- Satellites move fastest near periapsis and slowest near apoapsis.
- LEO satellites are relatively close to Earth and travel rapidly.
- LEO offers low communication delay and can provide high-resolution Earth observation.
- GEO satellites orbit approximately 35,786 km above Earth's surface.
- A geostationary satellite appears approximately stationary over one longitude.
- GEO is particularly useful for communications and continuous weather monitoring.
- Polar satellites can observe nearly the entire Earth as Earth rotates beneath their orbital paths.
- Sun-synchronous orbits provide similar lighting conditions on repeated observations.
- MEO is widely useful for navigation satellite constellations.
- Highly elliptical orbits can provide long periods of coverage over high latitudes.
- LEO and polar are not mutually exclusive; a satellite can be both.
- Elliptical transfer orbits allow spacecraft to move between different orbital altitudes.
- Different missions require different orbital characteristics.
- Low altitude improves some forms of imaging and reduces communication distance but reduces instantaneous coverage.
- High altitude increases coverage but generally increases signal delay and launch requirements.
- Orbit selection is an engineering trade-off involving coverage, resolution, latency, energy, lifetime, debris risk and mission objectives.
- The most appropriate orbit is the one that best supports the satellite's intended mission.
Check Your Understanding
- Explain the main difference between a circular orbit and an elliptical orbit.
- Define periapsis and apoapsis.
- Explain why a satellite in an elliptical orbit moves faster near periapsis than near apoapsis.
- What does orbital eccentricity describe? What is the eccentricity of a circular orbit?
- Compare LEO and GEO in terms of altitude, orbital speed, period, coverage and communication delay.
- Explain why a geostationary satellite must orbit above Earth's equator.
- Why are geostationary satellites particularly useful for communications and continuous weather monitoring?
- Explain how a polar satellite can eventually observe almost the entire Earth even though it follows approximately the same orbital plane during each revolution.
- Why are Sun-synchronous orbits useful for comparing images of Earth's surface taken on different days?
- A mission needs high-resolution images of forests around the world under similar lighting conditions. Recommend an orbit and justify your choice.
- A satellite must continuously monitor tropical storms over the same large region. Recommend an orbit and explain your reasoning.
- Explain why navigation systems often use constellations of satellites rather than one satellite.
- Describe how an elliptical transfer orbit can be used to move a spacecraft from a low circular orbit to a higher circular orbit.
- Explain why a highly elliptical orbit might be preferable to GEO for communications with very high-latitude regions.
- A company proposes putting every type of satellite into geostationary orbit because "GEO covers more of Earth, so it must always be better." Evaluate this proposal using at least four factors that engineers must consider when selecting an orbit.