Calculate request reply and reflection delays in space
Count every stated signal-path leg and add a separately supplied waiting time.
Exam scope and assessment notes
Core and Extended · Physics 0972, 2026–2028. Use supplied distances, periods and stated orbital models. Drawings are not to scale. Written practice is self-checked. Human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0972 has the same subject content as IGCSE 0625, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Calculate a one-way light delay.
- Add elapsed time intervals.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How many propagation legs does a command with a returned reply contain?
What you will learn
- Count every stated signal-path leg and add a separately supplied waiting time.
- Separate propagation legs, remote waiting and one-way separation in a signal budget.
Draw the path before calculating
A command from Earth to a spacecraft and its immediate reply require two one-way signal legs. If separation is stated constant at d and signals propagate at c, the minimum round-trip time is 2d/c. A reflecting radar signal also makes an outward and return journey.
A processing or waiting time at the remote object adds to the propagation delay. Do not hide that waiting time inside the speed or double it unless the problem explicitly gives two waits. A clocked round trip measures the combined route rather than one-way arrival.
Recover separation from an echo carefully
For a supplied constant-separation, negligible-wait radar model, d = ct/2 where t is total return time. Dividing by two is necessary because c times t gives the complete outward-and-return path. A one-way broadcast time would instead give d = ct.
Real spacecraft and planets move while signals travel. This course’s numerical tasks state fixed separation and neglect that motion where appropriate. Without these assumptions, a round-trip path may have unequal legs and more information is needed.
Pause and explain
A no-wait echo returns after 12 s in a fixed-separation model. What gives separation?
Worked example
A spacecraft at fixed supplied distance 9.0 × 10¹⁰ m replies after 40 s processing. Find total command-to-reply time.
Show the worked solution
- One-way propagation is 300 s.
- Two legs take 600 s.
- Add the single 40 s processing interval.
Answer 640 s, or 10 min 40 s.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
For a fixed separation 3.0 × 10¹⁰ m, find the no-wait request–reply delay.
Give me a hint
Compute the outward time and include the same returning path.
Compare my reasoning
- One way is 100 s.
- There are two equal legs.
- The total is 200 s.
200 s.
Look for these in your work
- Count both legs.
- State the no-wait fixed-separation assumptions.
At fixed supplied separation 6.0 × 10¹⁰ m, a reply waits 25 s. Find total delay.
Give me a hint
Add the wait once after counting both propagation legs.
Compare my reasoning
- Each leg is 200 s.
- Propagation totals 400 s.
- Add 25 s to obtain 425 s.
425 s, or 7 min 5 s.
Look for these in your work
- Include both legs.
- Add the single wait once.
A supplied no-wait radar echo returns after 8.0 s. Estimate the object’s fixed separation.
Give me a hint
Light traverses the separation twice before returning.
Compare my reasoning
- Total path is 3.0 × 10⁸ × 8.0 = 2.4 × 10⁹ m.
- Divide by two.
- The one-way separation is 1.2 × 10⁹ m.
1.2 × 10⁹ m.
Look for these in your work
- Distinguish path length from separation.
- Retain the fixed-distance model.
Common mistakes
- Count paths before multiplying by two.
- Do not double a single remote waiting interval.
- ct is the round-trip path, not the one-way separation.
What can you explain now?
A no-wait echo returns after 12 s in a fixed-separation model. What gives separation?
Compare with the explanation
c × 12 / 2
The elapsed time covers both outward and returning light, so half the total path gives separation.
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Make it stick
Separate propagation legs, remote waiting and one-way separation in a signal budget.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.